Who came up with the bizarre abuse + drowning theory? We will never know because the communication between lawyer and client is privileged. While counsel is not permitted to propose a theory contrary to known fact, the client can ask, "what factual scenario would help get me out of this mess." Then, "Yeah! She drowned." Then Q: "How do I explain not simply going to the police then?" A: "Your mental state would be a primary consideration. For example . . . . in Stave vs. Rumplestiltskin . . . " You get the idea. From what I have seen of JB, he would not hesitate to play it very close to the edge - trying to avoid libelous statements here.
Now, why would they cook up this kind of scheme? Theory one has been talked to death: Throw enough mud and see what sticks? Confuse a jury of twelve people who can't seem to figure out how to get out of jury duty? My law school Criminal Procedure prof was a legendary prosecutor in California. He said, "No theory or statement of facts is so ridiculous that constant repetition will not make it begin to sound reasonable."
But there is one more oddball possibility. As things got worse, could dad be convinced, under a grant of immunity (!), to agree with all this? The death is an accident, dad gets shame but no time, and Casey does a few years for child neglect and misleading the investigators. Why throw in the abuse charge? Had to mitigate all the lying somehow. On the whole it doesn't make sense that they would ever dream GA would confess to this - even under immunity, but being a dad, I can imagine sacrificing a great deal to minimize the impact on a daughter who had done something wrong. Something as heinous and cruel as this? Can't tell you because it hasn't happened.
